The invention relates to processing high temperature superconductor surfaces.
High Temperature Superconductor (HTS) thin films are intrinsically not smooth. HTS thin films deposition is made mostly by four different processes: pulsed laser ablation, magnetron sputtering, vapor phase evaporation, and metal organic chemical vapor deposition (MOCVD). In order to promote the epitaxy of the HTS film on a selected substrate which in turn provides the template of the crystalline growth, substrates are usually kept at an elevated temperature during HTS film deposition. Due to the nucleation and growth, islands or mesas are formed on the substrates to produce a rough surface. Typical peak to valley surface height variation ranges from 10 nm to 30 nm, and interpeak distance ranges from 0.3 micron to 0.6 micron.
For the fabrication of high temperature superconductor thin film devices, especially digital circuits or HTS interconnects in multi-chip modules (MCM), a smooth surface is crucial to the reliability and yield of the fabrication process. A rough surface is detrimental to the growth and performance of additional layer(s) on top of the film and becomes a reliability and yield killer to the large scale production of thin film devices.
There are many different ways to planarize a surface. For example, a non-smooth surface can be mechanically polished by lapping the surface with or without filler material. A rough surface, or a surface with steps, lines or structures can be planarized by first filling the surface with photoresist, spin-on-glass, or some polymer, then etched back chemically, or etched back by use of ion beam sputtering. A rough surface can also be smoothed by ion beam sputtering of the surface at a glancing angle within 4 degrees from the surface.
All existing methods have their problems. Mechanical lapping, and filling subject the surface to different materials which complicate the process, and detract from the yield in IC processing. Glancing angle sputtering is slow and difficult to control. Due to the above mentioned problems on existing planarization methods, HTS thin film devices are generally fabricated without surface smoothing procedures. As a consequence, devices have reliability and yield problems. The absence of a new method to planarize the HTS surface is impeding the progress of HTS thin film applications to devices.
It has been demonstrated that the bombarding effects of gas cluster ions on solid surfaces are quite different from those produced by monomer ion beams. Monomer ion beam sputtering is a well established technology in ion etching for surface layer removal, and in ion implantation. During monomer ion sputtering, the target atoms are removed layer by layer, a process that does not significantly alter the surface morphology. However, gas cluster ions contain hundreds or even thousands of atoms or molecules. The total energy of the cluster ranges from a few keV to a few tens of keV, which corresponds to several eV to several tens of eV per constituent atom. When the gas cluster hits the target surface, the collective motions of the cluster atoms during impact play dominant roles in the process kinetics. Computer simulation using molecular dynamics has shown the collective effect of gas cluster ion beams (GCIB) gives shallower, but massive damage and lateral sputtering. Due to the GCIB lateral sputtering effect, surfaces of Au, Pt, Cu, poly-Si, SiO.sub.2, Si.sub.3 N.sub.4, Si, and other materials, can be smoothed.
All conventional GCIB applications to date have involved semiconductors and metals. Demonstrations of shallow ion implantation, surface smoothing, high rate sputtering, and thin film deposition have been made. All cases demonstrated so far are simple systems involving single elements such as poly-Si, or diamond, Au, Pt, Cu, or binary compounds such as SiO.sub.2 or Si.sub.3 N.sub.4. High temperature superconductors such as Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 (YBCO) involve 13 atoms per unit cell of four different elements, and are very complicated as compared to the simple mono-element and binary compounds.